Occult Balls (オカルトボール Okarutobōru), also called Power Stones by some characters, are mystical objects gathered from Mystery Spots in the Outside World, and are the initial cause of the spread of Urban Legends in Gensokyo. They are central to the plot of Urban Legend in Limbo and are used as the main gameplay gimmick. Gathering the balls in battle triggers stage effects or enables the use of a Last Word.
The balls were originally scattered around the world hidden in Mystery Spots and later recollected by Sumireko Usami, who spread them in Gensokyo. Sumireko also spread the rumor that they could grant a wish to whoever collected seven of them, but this was untrue. Rather, because they are objects from the Outside World that defy common sense, they have the potential to get through the Great Hakurei Barrier, which is founded on a boundary of common sense. Sumireko uses this to temporarily enter Gensokyo, and for the same reason, whoever gathered seven of the balls in Gensokyo would be briefly transported to the Outside World.
Toyosatomimi no Miko eventually learns the balls cannot be activated within Senkai, and that the Lunar Capital ball was particularly powerful. Reimu also realizes that the Lunar Capital ball was not like the others since it did not come from the Outside World, and may have been put there by someone else without Sumireko's knowledge. In Legacy of Lunatic Kingdom, it is then revealed that the Lunar Capital ball was created by Sagume Kishin to enact the "Lunar Capital Transfer Plan", and that its power was also the initial cause of urban legends materializing in Gensokyo. By materializing the urban legends, if the rumor that "NASA is hiding the truth of what they saw on the Moon" was spread, the Lunar Capital could materialize inside Gensokyo.
It is said that more Occult Balls exist, but only seven are explicitly shown: